1. Field of the Invention
The present invention relates to the field of non-aqueous electrochemical cells which contain highly corrosive chemical components and so require hermetic sealing. More particularly, the invention concerns improvement in the hermetic seals of such cells which reduces corrosion and allows the electrical connector bridging the seal or pin terminal to be either positive or negative.
2. Description of the Prior Art
Non-aqueous electrochemical cells and particularly lithium batteries contain highly reactive components including, for example, such species as lithium, lithium salts, SO.sub.2, SOCl.sub.2, and SO.sub.2 Cl.sub.2. Packaging these materials properly is critical, since corrosion can readily destroy the most efficient battery through by-products and leakage. For that reason, stainless steel has become the preferred packing materials for lithium batteries. Stainless steel, particularly 300 series stainless steels are not attached by the corrosive cell materials.
In addition, one other requirement in these high energy batteries is that the cells be hermetically sealed to prevent outside contamination of the cells and prevent leakage which can result in loss of performance and/or damage to the environment. Accordingly, efforts have been made to hermetically seal the battery about the pin terminal, using stainless steel, as mentioned above, and glass which has been found to be an impervious insulator suitable for sealing the battery and preventing shorting of the cell between the anode and cathode.
One such successful device is found in U.S. Pat. No. 4,358,514. That invention involves an hermetic seal (which also is described in detail with regard to FIG. 1 of this application) includes a stainless steel header sized to hermetically fit the body and a glass annular ring bonded to the header. A terminal pin is bonded to the ring with the pin consisting of a solid central core of a material selected from nickel and nickel alloys and a stainless steel sleeve on the core.
That header device permits a hermetic seal between the header, the terminal pin and the body of the cell which does succeed in achieving a good glass-to-metal seal because the glass has a coefficient of expansion close enough to that of the nickel core over the contemplated temperature range. It has certain drawbacks, however. This is true, particularly with respect to corrosion problems which limit the use of such a terminal to the negative terminal only.
The prior art device is based on the premise that nickel must be used for the pin as it must take up the bulk of the expansion/contraction and the thermal coefficient of expansion of nickel closely follows that of the glass. However, the nickel, if made positive, is susceptible to anodic corrosion in the cell environment particularly with respect to the presence of SO.sub.2, SOCl.sub.2, SO.sub.2 Cl.sub.2 and other highly reactive solvent depolarizers commonly used in such cells. Therefore, it has heretofore been necessary to make the pin terminal of the cell negative so that it is then protected by the more reactive lithium or other alkali metal in the cell which preferentially decomposes.